What’s the difference between invalid and undeliverable?

You're sending to someone and the email bounces back with an error. But did it bounce because the address was never valid in the first place, or because it was valid but the server rejected it this time? That's the difference between invalid and undeliverable, and it matters more than it sounds.

An invalid address fails basic email rules before it even reaches a mail server. Think of it like sending a letter to "123 Main" with no city or state. It doesn't follow the standard format. Invalid addresses break RFC 5322 syntax rules (the official email address format), or the domain doesn't exist at all. You'd catch these with validation before you ever hit send.

Undeliverable is different. The address looks right on paper. It passes syntax checks. But when your server tries to deliver, the receiving server says "no." Maybe the mailbox doesn't exist anymore. Maybe the server is down. Maybe it's a spam trap now. You get an error code like "5.1.1 User unknown" or a temporary rejection. The address was valid when you added it. It's not now.

Here's why this matters for your sending. Invalid addresses should never hit your server in the first place. Undeliverable addresses need to be removed after a few tries, but a single rejection isn't always permanent. A temporary error (4xx code) might resolve tomorrow. A permanent one (5xx code) is done. Use hard bounce handling for undeliverables and focus your validation on catching invalids upfront.